Fiberoptic telecommunication systems are continually subject to demand for increased bandwidth. One way that bandwidth expansion has been accomplished is through dense wavelength division multiplexing (DWDM) wherein multiple separate data streams exist concurrently in a single optical fiber, with modulation of each data stream occurring on a different channel. Each data stream is modulated onto the output beam of a corresponding semiconductor transmitter laser operating at a specific channel wavelength, and the modulated outputs from the semiconductor lasers are combined onto a single fiber for transmission in their respective channels. The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Improvements in fiber technology together with the ever-increasing demand for greater bandwidth will likely result in smaller channel separation in the future.
Transmitter lasers used in DWDM systems have typically been based on distributed feedback (DFB) lasers operating with a reference etalon associated in a feedback control loop, with the reference etalon defining the ITU wavelength grid. Statistical variation associated with the manufacture of individual DFB lasers results in a distribution of channel center wavelengths across the wavelength grid, and thus individual DFB transmitters are usable only for a single channel or a small number of adjacent channels. Continuously tunable external cavity lasers have been developed to overcome this problem.
The advent of continuously tunable telecommunication lasers has introduced additional complexity to telecommunication transmission systems. In most telecommunication laser transmitters, the entire laser transmitter device is mounted on a single, common, high thermal conductivity substrate or platform, which is subject to thermal control with one or more TECs (thermoelectric controllers). Temperature control allows for maintenance of thermal alignment of all components. Without thermal control, spatial mis-alignment of optical components may arise, to due expansions and contractions associated with the various components, which will reduce wavelength stability, laser output power, and generally reduce the performance of the laser.
The application of thermal control to every component in a laser transmitter is often non-optimal. Indiscriminate thermal control of all components requires a substantial amount of power to provide cooling to the entire assembly, and unnecessarily increases laser operating cost due to power consumption. Indiscriminate thermal control can also result in introduction of thermal dissipation problems in the environment surrounding the laser. In many laser configurations, certain laser components are less susceptible to thermal mis-alignment, or are not susceptible to thermal mis-alignment problems, and providing thermal control to such components results in unnecessary power consumption. Heretofore, no laser systems have been available which provide selective thermal control for important optical components. This lack has resulted in increased operation costs and improved performance for such lasers.
The present invention relates to external cavity laser devices and methods wherein selective thermal control is applied to optical components having a greater susceptibility to thermal mis-alignment, and wherein unnecessary thermal control of other laser components is avoided. The apparatus of the invention comprises an optical output module for thermally controlling a gain medium and selected optical output components on a common, thermally controlled substrate or base. The methods of the invention comprise selectively cooling a gain medium and selected optical output components of an external cavity laser apparatus on a common, thermally controlled substrate or base.
The apparatus of the invention is configured so that thermal control of only the most alignment-sensitive components of an external cavity laser is required. Thus, the laser device and the cooling system can be configured to provide thermal control to only the most important, alignment- and temperature-sensitive elements of the external cavity laser. The portions that are not temperature sensitive are separately mounted on a different substrate or substrates that are remote or thermally isolated from the temperature controlled substrate. The external cavity laser may be tunable by various mechanisms to allow transmission at multiple selectable wavelength channels. Unnecessary thermal control of tuning mechanism components is avoided by the invention.
The external cavity laser of the invention may comprise a gain medium having first and second output facets, and an end mirror. The gain medium emits a first coherent beam from the first output facet along a first optical path, and a second coherent beam from the second output facet along a second optical path. The end mirror is positioned in the first optical path and is optically coupled to the first output facet of the gain medium. An optical output assembly or module may be positioned in the second optical path and optically coupled to the second output facet of the gain medium. The end mirror and the second output facet define an external cavity, such that the gain medium is within the external cavity and subject to receiving optical feedback from the external cavity.
A thermally conductive substrate is provided in which the gain medium and the optical output assembly are mounted. The thermally conductive substrate is engineered to have high thermal conductivity and a coefficient of thermal expansion that is matched to that of the gain medium. The gain medium and the components of the optical output assembly are temperature sensitive components, and mounting of these components on a common substrate having a high coefficient of thermal conductivity allows for selective and accurate temperature control and cooling of the components of the output assembly.
A thermoelectric controller (TEC) may be joined or coupled to the thermally conductive substrate that provides thermal control for the thermally conductive substrate. The gain medium and optical output assembly are configured to be thermally coupled to, and thermally controlled by, the thermoelectric controller via thermal conduction through the substrate. The thermoelectric controller of the present invention allows for the gain medium and the output assembly to be thermally controlled independently from the end mirror and other components of the external cavity laser.
Temperature monitoring of the thermally conductive substrate is provided by a thermistor that is operatively coupled to the thermoelectric controller and to the thermally conductive substrate, thus allowing the thermistor to detect temperature changes in the substrate (and hence the thermally coupled gain medium and optical output assembly). If any temperature deviation from the optimum temperature is detected, the thermistor signals the thermoelectric controller to adjust the temperature of the substrate so as to maintain a selectable optimal temperature.
The end mirror of the external cavity laser may be mounted on a second substrate that is thermally isolated from the thermoelectric controller. In certain embodiments, the end mirror and other components associated with the external cavity may be mounted together on the second substrate or on a plurality of substrates which are distinct or remote from, or otherwise thermally isolated with respect to the thermally conductive substrate supporting the gain medium and the optical output module.
A first collimating lens may be included on the substrate, and positioned in the first optical path to collimate the coherent beam emitted along the first optical path towards the end mirror. The optical output assembly may comprise a second collimating lens optically coupled to the second output facet of the gain medium, with the second collimating lens positioned in the second optical path proximate to the second output facet of the gain medium. The second collimating lens of the optical output assembly, like the gain medium and first collimating lens, is also mounted on the thermally conductive substrate and configured to be thermally controlled by the thermoelectric controller via thermal conduction through the substrate.
The optical output assembly may also comprise a fiber focusing lens positioned in the second optical path and operatively coupled to an optical fiber. The optical output assembly may also comprise an optical isolator that is positioned in the second optical path and optically coupled to the second collimating lens. In some embodiments, the optical isolator is positioned in the second optical path after the second collimating lens and before the fiber focusing lens, such that the fiber focusing lens is optically coupled to the optical isolator. The optical isolator, which provides unidirectional transmission of light from the gain medium to the fiber, is not particularly alignment sensitive but, due to its location adjacent to the alignment collimating lens and fiber focusing lens, may conveniently be located on the thermally controlled substrate.
Components of the external cavity laser which are remote from or otherwise thermally isolated from the thermally conductive substrate and its temperature controller may comprise, for example, a channel selector and a tuning assembly. The channel selector, which may comprise a wedge etalon, grating, electro-optic etalon, graded filter or other wavelength tuning device, may be positioned in the first optical path between the first output facet of the gain medium and the end mirror. The tuning assembly is operatively coupled to the channel selector and configured to adjust the channel selector via translational and/or rotational positional adjustment, voltage adjustment, or other form of tuning adjustment. The tuning assembly may comprise, for example, a stepper motor configured to positionally adjust a wedge etalon in the first optical path. The channel selector and the tuning assembly are positioned such that the thermally conductive substrate and gain medium and optical output assembly thereon are subject thermal control that it is independent or substantially independent from any thermal characteristics or thermal control associated with the channel selector and tuning assembly. In other words, the channel selector and the tuning assembly are thermally isolated from the thermally conductive substrate and the thermoelectric controller and are positioned remotely from the thermally conductive substrate.
In certain embodiments, the optical output assembly may include a coarse spectrometer that is usable for evaluating the output wavelength of the external cavity laser along the second optical path. The coarse spectrometer is mounted on the thermally conductive substrate and thermally coupled to the TEC. The coarse spectrometer may be positioned in the second optical path after the second collimating lens and before the optical isolator, or after the optical isolator and before the fiber focusing lens.
In an alternative embodiment, the optical output assembly may comprise a grid etalon wherein the grid etalon is mounted on the thermally conductive substrate and thermally coupled to the TEC therethrough. The grid etalon may be positioned in the second optical path after the second collimating lens and before the optical isolator, or after the optical isolator and before the fiber focusing lens.
Temperature sensitive elements which may be subject to selective thermal control on a single thermally conductive substrate in accordance with the present invention include, without limitation; the gain medium, the output coupling optics (collimating lenses), and the grid etalon (etalon). The grid etalon, while temperature sensitive, may in some embodiments be subject to independent temperature control on a separate substrate.
The invention may be embodied in a laser apparatus comprising a optical output module which itself comprises a gain medium and an optical output assembly mounted on a common, thermally conductive substrate and thermally coupled to a thermoelectric controller or other temperature control source. The optical output assembly may vary in configuration, but will generally be configured such that the optical output components are mounted on a thermally conductive surface to regulate the temperature of alignment sensitive output components.
The present invention also provides methods for selectively cooling an external cavity diode laser apparatus. The methods may comprise providing a gain medium having first and second output facets, an end mirror optically coupled to the first output facet, and a optical output assembly optically coupled to a second output facet, and thermally controlling the gain medium and the optical output assembly independently from the end mirror. Thermally controlling the gain medium and the optical output assembly may comprise mounting the gain medium and optical output assembly on a thermally conductive substrate, and coupling the thermally conductive substrate to a thermoelectric controller.